Internal combustion engines of the free-piston type are known and have been successfully manufactured as air compressors, gas generators for turbine engines, and as hydraulic pumps. These types may be called “conventional” free-piston engines. Difficulties have prevented the technology from expanding to other configurations. A large body of literature exists on the potential of using an internal combustion free-piston engine (FPE) to drive a linear electric generator. Attempts have also been made to convert the reciprocating action of a free-piston engine directly to rotational power by use of rack-and-pinion mechanisms. In this disclosure these types will be called “advanced” free-piston engines. Neither of these types, the electric generator or rack-and-pinion types, has been successfully implemented. Producing useful power levels from advanced designs has proved especially difficult. One reason for the under-powered performance of advanced FPE designs is that they require reciprocating plungers of significantly heavier weight than conventional FPE's.
FIG. 2 and FIG. 4 are presented here for explicating problems inherent with advanced free-piston engine design. FIG. 2 shows a plunger assembly of mass m reciprocating between a combustion chamber 50 and a bounce chamber 30. The following analysis applies to most all FPE configurations. For example, the plunger may reciprocate within two combustion chambers, or two plungers may operate opposed to each other. An opposed-piston FPE may have either a single combustion cylinder or dual combustion cylinders. FIG. 4 shows the force curve on bounce piston 16 and power piston 1 during the compression stroke, with the horizontal axis showing piston displacement. The graph is drawn for a chamber six units in length so that a piston displacement of five units produces a compression ratio (CR) of 6 to 1. The compression ratios at various piston displacements are marked on the force curve. The vertical axis shows normalized force units. Note that graph is broken into two sections and that the force scale at the top of the vertical axis is more compressed than the lower section.
Referring to FIG. 2 we consider the operation of the engine at idle with no load, and burning only enough fuel to overcome friction. The system operates as a spring-mass oscillator with a gas-spring at each end of the reciprocating mass. Consider the idling engine operating with stroke length SL that produces a 7.5 to 1 compression ratio. Lines TC (top-center) and BC (bottom-center) mark the outboard and inboard reversal points of piston travel, respectively. Point TC is marked on the force curve in FIG. 4. It can be observed that the curve's tangent at point TC, shown by the dashed line, is a good linear approximation of the force curve between compression ratios of 6:1 and 10:1. Therefore the system closely follows the behavior of a simple harmonic oscillator for stroke lengths within these compression ratios. The resonant frequency of the system, co, is known to have the formula:ω=√(K/m)  (1)where K is the system's spring constant. The value of K is expressed in the slope of the tangent line in FIG. 4.
Formula (1) makes evident that increasing values of m (mass) produce decreasing cycle rates. Furthermore, the idling cycle rate is the maximum cycle rate of the engine because linkage to a load only dampens and slows the oscillation regardless of fuel burn. FPE designs having heavy plunger weights thus tend to suffer low engine speeds and low power performance. If an automotive sized FPE is limited to a cycle rate of only a hundred or so cycles per minute, it is not capable of producing a power level comparable to conventional internal combustion engines.
In order to increase the inherently slow cycle rates of advanced FPE models researchers typically run the engines at very high compression ratios. As is evident in FIG. 4, the slope of the air-spring force curve, and therefore cycle rate of the system, increases dramatically for compression ratios above 30 to 1. For example, U.S. Pat. No. 6,199,519 (Mar. 13, 2001) describes the testing of an advanced FPE at Sandia National Laboratory. An acceptable engine speed of 40 Hz, equivalent to 2400 rpm, was attained but only at compression ratios as high as 40 to 1. U.S. Pat. No. 8,453,612 (Jun. 4, 2013) describes testing of an advance FPE at Stanford University at compression ratios between 30:1 and nearly 100:1. But such high compression ratios are not practical for an internal combustion engine.
Another method for increasing the cycle rate of a heavy mass FPE is to increase the engine's piston diameters. Increasing the piston diameters increase the spring constant K of the system's air-springs. Referring to FIG. 2, increasing the diameter of bounce piston 5 may be a practical design option. Bounce chambers can be enlarged without increasing pumping losses because no gas exchange takes place in the chamber. Furthermore, complex mixing and combustion processes do not occur in the bounce chamber. Both power and bounce piston diameters must be enlarged together, however, for this method to work. Compression ratios in the combustion chamber become extreme if the bounce piston diameter alone is increased. But to attain useful cycle rates at reasonable compression ratios this method yields unworkably large combustion chambers (50).
Control of the engine's stroke length has also proved problematic for advanced FPE's designed to run at standard compression ratios. The force curve shown in FIG. 4 reveals that, at standard engine compression ratios, piston stroke variation of one or two tenths of inch produces little force differential. That is, small variations in the oscillating system's energy produces large variations in stroke length. This trait leads to a common problem in advanced FPE's: piston head-strikes. A piston striking the combustion cylinder head is a damaging failure that must be prevented in practice. Researchers commonly attempt to control stroke length in advanced FPE designs, and avoid head-strikes, by implementing very precise stroke-by-stroke control of the engine's fuel injectors and combustion timing. At Sandia National Laboratory complex mechanisms have been used to inject high pressure helium or nitrogen gas into the bounce chambers, on a stroke-by-stroke basis, in order to control stroke length. These methods are complex and costly, and have not been sufficiently developed.
A further disadvantage of current advanced FPE designs is their fixed engine speed. The cycle rate of FPE's is determined by the resonant frequency according to formula (1) and varies very little with engine power. It would be desirable for an advanced FPE to be operable over a range of cycle rates while maintaining compression ratios at moderate levels. It would even be desirable for the engine to be switchable between two spring constants: one for low speed idle and engine starting and a second for high speed operation.
Associated with the control issues described above is the operation of the combustion chamber gas valves in advanced FPE's. Many designs use externally powered poppet valve mechanisms. This approach adds cost and complexity to the engine and reduces efficiency. Designs that operate valves directly from plunger movement suffer the disadvantage of fixed valve timing that is difficult to adjust. An additional issue associated with advanced FPE's involves the two-stroke engine cycle. Other than in large marine engines, the two-stroke engine cycle is commonly implemented in a manner that produces worse exhaust emissions and lower thermal efficiency than four-stroke engines. Current advanced FPE designs have not sufficiently addressed this issue.
Finally, advanced free-piston designs are usually limited to single cylinder configurations. Opposed-piston types are limited to two combustion chambers and reciprocating plungers. Additionally, cumbersome mechanical linkages are needed to synchronize the plungers in opposed-piston FPE's. Adding combustion cylinders to current advanced FPE models is usually accomplished by ganging together independently reciprocating units. The scheme suffers serious vibration issues because the motion of the plungers is unsynchronized. A multicylinder free-piston engine configuration in which the multiple plungers are synchronized would substantially increase power generation.